CMake: Don't set a default Python version.

A blank string to FindPythonInterp finds the version
of Python that the system "python" interpreter invokes,
so this means the default is now the system default.

dtool/Config.cmake

@@ -418,7 +418,7 @@ mark_as_advanced(ANDROID_NDK_HOME ANDROID_ABI ANDROID_STL
 #
 
 # Is Python installed, and should Python interfaces be generated?
-set(WANT_PYTHON_VERSION "2.7"
+set(WANT_PYTHON_VERSION ""
   CACHE STRING "Which Python version to seek out for building Panda3D against.")
 
 find_package(PythonInterp ${WANT_PYTHON_VERSION} QUIET)